Time and depth scales of fine sediment delivery into gravel stream beds: Constraints from fallout radionuclides on fine sediment residence time and delivery

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• 作者：J.D. Gartner^a ; ^{john.d.gartner.gr@dartmouth.edu} ; C.E. Renshaw^a ; W.B. Dade^a ; F.J. Magilligan^b
• 关键词：Stream sediment ; Residence time ; Hyporheic zone ; Fallout radionuclides ; Dams
• 刊名：Geomorphology
• 出版年：2012
• 期刊代码：31_0169555x
• 类别：soc
• 出版时间：15 May, 2012
• 卷：151-152
• 期：Complete
• 页码：39-49
• 文件大小：968 K

摘要

Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) ⁷Be (t_1/2 = 53 days) and ²¹⁰Pb_ex (t_1/2 = 22.3 years). We placed mesh cylinders filled with ⁷Be-free sediment into a stream bed to quantify the capture of ⁷Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410 km², and core depths ranged from 19 to 77 cm. The ²¹⁰Pb_ex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, ⁷Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to > 300 days in unregulated rivers. Cores from a regulated river are notably different鈥攕ubsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function.